1. Field of the Invention
The present invention relates to a powder pressing apparatus and a powder pressing method, and more specifically to a powder pressing apparatus and a powder pressing method for formation of a green compact by compressing a rare-earth magnetic powder filled in a cavity.
2. Description of the Related Art
Generally, a rare-earth magnetic powder pressing apparatus of the above kind comprises a die having a vertical through hole, an upper punch to be inserted from above the through hole of the die, and a lower punch inserted into the through hole of the die for sliding movement relative to the die. The lower punch provides a cavity within the through hole of the die. The cavity is filled with the rare-earth magnetic powder gravitationally supplied from above and then, the upper punch is inserted into the through hole so that the rare-earth magnetic powder is pressed by the upper punch and the lower punch into a compact.
According to the above pressing apparatus, generally, a lubricant (mold releasing agent) is applied to a side surface of the through hole for prevention of seizure of the side surface of the through hole as well as for ease of removing the compact out of the through hole. For this particular operation, there is an apparatus disclosed in Japanese Patent Laid-Open No. 3-291307, in which a nozzle is provided on a side surface of the lower punch. According to this apparatus, while the die is moving relative to the lower punch, the lubricant is supplied to the side surface of the through hole directly from the nozzle, thereby improving operating efficiency of the apparatus.
On the other hand, Japanese Utility Model Laid-Open 62-146597 discloses an apparatus in which a suction port is provided in the side surface of the through hole of the die or in a lower surface of the upper punch. When the powder is pressed into the compact, air and other gases within the cavity are sucked (deaerated), thereby decreasing the amount of the gases captured in the compact.
There are problems, however. According to the former disclosure (Japanese Patent Laid-Open No. 3-291307), although the lubricant can be applied reliably to a region of the side surface of the through hole of the die near the nozzle, it is difficult to make the application uniformly to the entire side surface of the through hole. As a result, it becomes difficult to reliably remove the compact from the through hole. If the application is to be made uniformly, on the other hand, then a large amount of the lubricant must be supplied, so that the compact is coated with an unnecessary amount of the lubricant. The excess amount of the lubricant will make a surface of the compact brittle, and therefore susceptible to a cracking or a flaking defect. Further, if the former conventional apparatus is used to press the rare-earth magnetic powder, the lubricant, which is an organic compound, is likely to increase carbon content of the compound after sintering, deteriorating magnetic characteristics of a rare-earth magnet. Furthermore the green compact comprised of the powder manufactured by a strip casting process has a poorer compact strength due to a sharp distribution curve of grain sizes.
According to the above pressing apparatus, the cavity is filled with the rare-earth pressing magnetic powder gravitationally supplied from above. This often causes the powder to be filled non-uniformly, particularly at a corner portion formed by the side surface of the through hole of the die and the lower punch. This problem is especially serious if the powder being used is manufactured by a strip casting process, because the powder made by the strip casting process has a much poorer fluidity than a mold casting process, characterized by a sharp distribution curve of grain sizes. Thus, an attempt may be made to apply the latter apparatus (Japanese Utility Model Laid-Open No. 62-146597) for making suction from the side surface of the through hole of the die while the powder is being filled into the cavity.
However, although the provision of a suction port in the side surface of the through hole of the die makes it possible to deaerate, it is difficult to form a large number of suction ports uniformly. Further, even if the suction is performed from the side surface of the through hole of the die, it is still difficult to improve uniformity of the powder filling, particularly at the corner portion formed by the lower punch and the side surface of the through hole of the die. Therefore, improvement is not achieved in quality or yield of the compact. As a result, improvement is not achieved in productivity, either.
It is therefore a primary object of the present invention to provide a powder pressing apparatus and a powder pressing method capable of improving the quality and yield of the compact, thereby improving the productivity.
According to an aspect of the present invention, there is provided a powder pressing apparatus for formation of a compact by pressing a powder, comprising: a die having a vertical through hole; a first punch and a second punch, each being vertically movable within the through hole relative to the die, for compression of the powder within a cavity formed in the through hole; and a supplying means including a supply port provided in a side surface of the first punch, for supply of a lubricant with a gas from the supply port to the side surface of the through hole within the through hole.
According to another aspect of the present invention, there is provided a method of pressing a powder for formation of a compact through. compression of the powder by a first punch and a second punch within a cavity formed in a vertical through hole of a die, comprising: a lubricant supplying step of supplying a lubricant with a gas from a side surface of the first punch to a side surface of the through hole within the through hole; a powder filling step of filling the cavity with the powder; and a press forming step of forming the compact by pressing the powder filled in the cavity by the first punch and the second punch.
According to the present invention, a lubricant with a gas is supplied in la form of a spray or mist for example, from a side surface of the first punch to a side surface of the through hole of the die. Therefore, the lubricant can be applied easily and uniformly to the side surface of the through hole of the die, applying only a relatively small amount of the lubricant. As a result, the compact can be reliably taken out of the through hole. Further, magnetic characteristics of a magnet obtained by sintering the compact can be improved since carbon content of the resulting magnet is small. As a result, it becomes possible to improve the quality and yield of the compact, thereby improving the productivity.
According to the present invention, preferably, the lubricant applied to the side surface of the through hole is spread by an absorbing member, for example. This makes it possible to apply the lubricant even more uniformly on the side surface of the through hole, making it possible to provide more saving of the amount of application.
Further, preferably, the first punch is formed to have a polyangular section so as to have an angular portion, and the lubricant is supplied from inside the first punch through near the angled portion to the side surface of the through hole. If the first punch is formed to have a polyangular section, a corner portion of the through hole corresponding to the angled portion becomes susceptible to non-uniform application. However, by supplying the lubricant from near the angled portion of the first punch, it becomes possible to reliably apply the lubricant to the corner portion of the through hole. As a result, the compact can be reliably taken out of the through hole of the die even if the first punch has the polyangular section, making it possible to improve further the quality and yield of the compact, thereby improving the productivity, as well as expanding the life of the metal molds.
Further, preferably, the lubricant is supplied to the side surface of the through hole, while a gap between the side surface of the first punch and the side surface of the through hole is sealed by a sealing portion for example provided in the side surface of the first punch farther away from the cavity than is the supply port. If the first punch is a lower punch, the lubricant is reliably discharged upward by the sealing provided at the lower portion of the supply port. As a result, a body of powder captured between the side surface of the lower punch and the side surface of the through hole of the die can be removed by the upward blast provided by the lubricant with gas. Therefore, it becomes possible to reduce wear on the side surface of the lower punch and the side surface of the through hole of the die caused by the powder. This advantage becomes especially significant if the powder contains a highly abrasive rare-earth magnetic powder. Further, the applied lubricant can be prevented from flowing downward. On the other hand, if the first punch is an upper punch, the lubricant can be reliably discharged downward by sealing an upper portion of the supply port, and the applied lubricant can be preventing from leaking through a gap between the upper punch and the side surface of the through hole to the upper surface of the die and so on.
Further, preferably, the first punch is the lower punch. As described above, according to the present invention, the lubricant can be applied easily and uniformly to the side surface of the through hole of the die, by using a relatively small amount of the lubricant. As a result, there is a very rare possibility for the lubricant to pool on the upper surface of the lower punch. Thus, the powder is easily set into form when pressed, and therefore the compact having desired characteristics can be obtained. Even if the lower punch has the upper surface formed into a recess, there is still a very rare possibility for the lubricant to pool on the upper surface of the lower punch, and therefore the same advantage is achieved.
Further, preferably, the powder contains a rare-earth magnetic powder. According to the present invention, application of an unnecessarily large amount of the lubricant to the compact can be prevented. Thus, even if the compact is formed from the powder containing the rare-earth magnetic powder, and if this compact is sintered, a resulting rare-earth magnet has a low carbon content. As a result, magnetic characteristics of the rare-earth magnet can be improved.
According to another aspect of the present invention, there is provided a powder pressing apparatus for formation of a compact by pressing a powder, comprising: a die having a vertical through hole; an upper punch and a lower punch, each being vertically movable within the through hole relative to the die, for compression of the powder within a cavity formed in the through hole; and a sucking means including a suction port provided in a side surface of the lower punch, for providing suction from the cavity under a reduced pressure from the suction port while the cavity is being filled with the powder.
According to still another aspect of the present invention, there is provided a method of pressing a powder for formation of a compact through compression of the powder by an upper punch and a lower punch within a cavity formed in a vertical through hole of a die, comprising: a powder filling step of filling the cavity with the powder while the cavity is under suction by a reduced pressure provided from a side surface of the lower punch; and a press forming step of forming the compact by pressing the powder filled in the cavity by the upper punch and the lower punch.
According to the present invention, the cavity is filled with the powder while the cavity is under suction by a reduced pressure provided from the side surface of the lower punch. Thus, the powder can be reliably filled even to the corner portion formed by the side surface of the through hole of the die and the lower punch, making it possible to prevent non-uniform filling of the powder in the cavity. Thus, it becomes possible to improve the quality and yield of the compact, thereby improving the productivity.
According to the present invention, preferably, filtration is provided by a filtering member provided in the side surface of the lower punch for covering the sucking port for example, during the suction from the cavity by a reduced pressure for preventing the lower punch from sucking the powder. This prevents clogging of the suction port, making possible to provide the suction reliably and uniformly.
Further, preferably, the lower punch is formed to have a polyangular portion so as to have an angled portion, and the cavity is sucked by a reduced pressure provided from near the angled portion. If the lower punch is formed to have a polyangular section, a corner portion of the through hole corresponding to the angled portion becomes susceptible to non-uniform filling. However, through providing the suction of the cavity by the reduced pressure from near the angled portion of the lower punch, such a corner portion can also be filled reliably with the powder, making it possible to prevent non-uniform filling reliably. As a result, it becomes possible to further improve the quality and yield of the compact, thereby improving the productivity.
Still further, preferably, the gap between the side surface of the lower punch and the side surface of the through hole of the die is sealed by a sealing portion provided in the side surface for the lower punch below the section port for example, while the cavity is under the suction provided by the reduced pressure. This makes it possible to generate a partial vacuum above the lower punch while the cavity is under suction, thereby improving the effect of the suction.
As has been described here above, non-uniform filling of the powder within the cavity is prevented. Thus, even if the powder includes a rare-earth magnetic powder having a poor fluidity, it becomes possible to fill the corner portion of the through hole with the powder, making it possible to form the compact of good quality. Therefore, the present invention is effective if the powder includes the rare-earth magnetic powder. The present invention is specially effective if the rare-earth magnetic powder is manufactured by a strip casting process, thus having a poor fluidity.
According to another aspect of the present invention, there is provided a powder pressing apparatus for formation of a compact by pressing a powder, comprising: a die having a vertical through hole; a first punch and a second punch, each being vertically movable within the through hole relative to the die, for compression of the powder within a cavity formed in the through hole; a lubricant holding means provided in a side surface of the first punch, for application of a lubricant to a side surface of the through hole in contact with the side surface of the through hole; and a lubricant supplying means for supply of the lubricant to the lubricant holding means.
According to still another aspect of the present invention, there is provided a method of pressing a powder for formation of a compact through compression of the powder by a first punch and a second punch within a cavity formed in a vertical through hole of a die, comprising: a lubricant supplying step of supplying a lubricant to a side surface of the through hole by bringing a lubricant holding means holding the lubricant into contact with the side surface of the through hole; a powder filling step of filling the cavity with the powder; and a press forming step of forming the compact by pressing the powder filled in the cavity by the first punch and the second punch.
According to the present invention, the lubricant can be applied easily and uniformly to the side surface of the through hole of the die, applying only a relatively small amount of the lubricant. As a result, the compact can be reliably taken out of the through hole. Further, magnetic characteristics of a magnet obtained by sintering the compact can be improved since carbon content of the resulting magnet is small. As a result, it becomes possible to improve the quality and yield of the compact, thereby improving the productivity.
Further, preferably, the powder contains a rare-earth magnetic powder. According to the present invention, application of an unnecessarily large amount of the lubricant to the compact can be prevented. Thus, even if the compact is formed from the powder containing the rare-earth magnetic powder, and if this compact is sintered, a resulting rare-earth magnet has a low carbon content. As a result, magnetic characteristics of the rare-earth magnet can be improved.
The above described objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of embodiments when taken in conjunction with the accompanying drawings.